This research is a continuation of field studies of rhesus monkey populations in India which were begun in October 1959. The studies are starting their 17th year, and this proposal requests three additional years of support to enable the project to complete 20 years of study. As such, it will produce a continuous census record of a natural primate population covering four generations of rhesus. The field work involves accurate censuses of rhesus groups in northern India three times a year. The groups are natural, free-ranging groups in a rural agricultural area of approximately 300 square miles around the city of Aligarh, 80 miles southeast of New Delhi. Data are obtained on total population, group sizes, sex and age ratios, infant mortality, juvenile and adult losses, annual natality, annual production, population trends, and human-monkey interactions. We also propose to sample approximately 150 other rhesus groups throughout Uttar Pradesh and compare their present status with their known status in 1959-60 and 1964-65. This will test the applicability and validity of the Aligarh data to a broader sample of the rhesus populations of northern India. This study will provide a long-term assessment of population changes in rhesus monkeys in an area typical of the upper Gangetic Plain near the center of rhesus distribution. The data are important in evaluating the supply of primates for biomedical research.